1. Preventing Overgrowth among Tree Branches
exposed = unprotected
with no warning = without any indication beforehand
congestion = overcrowding
2. Crown of Thorns Starfish and Coral Reefs
outbreak = sudden increase
accompany = occur along with
converge = come together
severity = seriousness
The crown of thorns starfish, Acanthaster Tlanci, is large, twenty-five to thirty-five centimeters in diameter, and has seven to twenty-one arms that are covered in spines. It feeds primarily on coral and is found from the Indian Ocean to the west coast of Central America, usually at quite low population densities. Since the mid-1950s, population outbreaks at densities four to six times greater than normal have occurred at the same time in places such as Hawaii, Tahiti, Panama, and the Great Barrier Reef. The result has often been the loss of a fifty percent to nearly one hundred percent of the coral cover over large areas.
A single Acanthaster can consume five to six square meters of coral polyps per year, and dense populations can destroy up to six square kilometers per year and move on rapidly. Acanthasters show a preference for branching corals, especially Acroporids. After an outbreak in a particular area, it is common to find that Acroporids have been selectively removed, leaving a mosaic of living and dead corals. In places where Acroporids previously dominated the community devastation can be almost complete, and local areas of reefs have collapsed.
Areas of dead coral are usually colonized rapidly by algae and often are later colonized by sponges and soft corals. Increases in abundance of plant-eating fish and decreases in abundance of coral-feeding fish accompany these changes. Coral larvae settle among the algae and eventually establish flourishing coral colonies. In ten to fifteen years the reefs often return to about the same percentage of coral cover as before. Development of a four-species diversity takes about twenty years.
Two schools of thought exist concerning the cause of these outbreaks. One group holds that they are natural phenomena that have occurred many times in the past, citing old men's recollections of earlier outbreaks and evidence from traditional cultures. The other group maintains that recent human activities ranging from physical coral destruction through pollution to predator removal have triggered these events.
One theory, the adult aggregation hypothesis, maintains that most species is more abundant than we realize when a storm destroys coral and causes a food shortage. The adult Acanthasters converge on remaining portions of healthy coral and feed hungrily. Certainly there have been outbreaks of Acanthaster following large storms, but there is little evidence that the storms have caused the enough reef damage to create a food shortage for these starfish.
Two other hypotheses attempt to explain the increased abundance of Acanthaster after episodes of high terrestrial runoff following storms. The first hypothesis is that low salinity and high temperatures favor the survival of the starfish larvae. The second hypothesis emphasizes the food web aspect, suggesting that strong fresh water runoff brings additional nutrients to the coastal waters, stimulating phytoplankton production and promoting more rapid development and better survival of the starfish larvae.
Those favoring anthropogenic (human influenced) causes have pointed to the large proportion of outbreaks that have been near centers of human populations. It has been suggested that coral polyps are the main predators of the starfish larvae. Destruction of coral by blasting and other bad land use practices would reduce predation on the starfish larvae and cause a feedback in which increases in Acanthaster populations cause still further coral destruction. Unfortunately, there are too few documented instances of physical destruction of coral being followed by outbreaks of Acanthaster for these hypotheses to be fully supported.
Another group of hypothesis focuses on removal of Acanthaster's predators. Some have suggested that the predators might have been killed off by pollution whereas others have suggested that the harvesting of vertebrate and invertebrate predators of Acanthaster could have reduced mortality and caused increased abundance of adults. The problem with this group of hypothesis is that it is difficult to understand how reduced predation would lead to sudden increases in Acanthaster numbers in several places at the same time in specific years. It seems probable that there is no single explanation but that there are elements of the truth in several of the hypotheses. That is there are natural processes that have led to outbreaks in the past, but human impact has increased the frequency and severity of the outbreaks.
allowed for = made possible
harsh = severe
duration of sea ice = length of time the ice lasts
annihilated = destroyed
4.Changes in the Amount of Forest on the Great Plains
本篇属于比较规律的一篇生物文章，从森林变化这种现象着手，进而分析背后的原因; 建议大家多多了解相关的背景知识，比如 TPO 09：The Arrival of Plant Life in Hawaii; TPO 25：The Evolutionary Origin of Plants，从这些文章中我们可以知道植物的产生和进化原理，并且之后背后的原因。每种植物的生长和进化套路相似，所以大家可以仔细分析一下这两篇文章的结构，总结类似的文章结构规律和出题特征。
文章内容：最初被常绿林覆盖的森林，由于气候变得温暖干燥，变成了落叶林，最后成了草地。在一些山脊之类的地方树木比较占优势，一方面山顶气候凉爽并且接收到更多的降水(precipitation)，另一方面，山脊是天然的防火地带(natural firebreak)，但草原被烧毁后依旧能够拥有“野火烧不尽”的势头 。然而在大平原地区，森林减少后，接下来又经历了逆袭，数量开始增多，具体的原因主要在于人类的活动，随着农牧业的发展，草原数量逐渐减少，火灾频率也随之降低，所以，森林又得以回归 。
dense = thick
eased into = slowly entered
retain = keep
inevitable = certain
5.How Plants and Animals Arrived in the Hawaiian Islands
6. Early Modern Industrialization
determinants = causes
hastened = rushed
diffusion = dispersal
perfected = completed
7. Birds and Food Shortage
duration of = length of
enables = allows
in some degree = to a certain extent